THE GERM-PLASM THEORY 413 



ill other words, they alone contained the indispensable germ-plasm. 

 With the help of Figs. 94 and 95 I hope to be able to make this 

 matter clear. 



In the hydroid polyps and their medusoids the germ-cells always 

 arise in the ectoderm ; in species which produce sexual medusoids by 

 budding, the germ-cells arise in the ectoderm of the manubrium of 

 these medusoids (Fig. 94, M, hz). But in many species these sexual 

 stages have degenerated in the course of phylogeny into so-called 

 gonophores, that is, to medusoids which still exhibit more or less 

 complete bells, but neither mouth (m) nor marginal tentacles (T), and 

 which no longer break away from the colony to swim freely about, 

 to feed independently, and to produce and ripen germ-cells. The 

 degeneration of the 'gonophores' often goes even further; in many 

 the medusoid bell is represented only by a thin layer of cells, and in 

 some even this token of descent from medusoid ancestry is absent, 

 and they are mere single-layered closed brood-sacs (Fig. 95, Gph). 



The adherence of the sexual animal to the hydroid colony has, 

 however, made a more rapid ripening of the germ -cells possible, and 

 nature has taken advantage of this possibility in all the cases known 

 to me, for the germ-cells no longer arise in the manubrium of the 

 mature degenerate medusoid, that is, of the gonophore, but earlier, 

 before the bud which becomes a gonophore possesses a manubrium. 

 The birthplace of the germ -cells is thus shifted back from the manu- 

 brium of the medusoid to the, young gonophore-bud (Fig. 94, M, hz). 

 The same thing occurs in species in which the medusoids are liberated, 

 but live only for a short time, for instance, in the genus Podocoryne. 

 Although perfect medusoids are formed, these have their germ-cells 

 fully developed at the time of their liberation from the hydroid 

 colony. But in species in which the medusoid-buds have really 

 degenerated and are no longer liberated, the birthplace of the germ- 

 cells is shifted even further hack, and in the first place into the stalk 

 (>S'^, kz") of the polyp from the gonophore-buds. This is the case 

 in the genus Hydractinia. In the further course of the process 

 the birthplace of the germ-cells has shifted as far back as to the 

 branch from which the polyp has grown out (Fig. 94, A, kz") ; and 

 finally, in the cases in which the medusoid has degenerated to a mere 

 brood-sac (Fig. 95, Gijh), even to the generation of polyps immediately 

 before, that is, into the polyp-stem from which the branch arises that 

 bears the polyps producing the gonophore-bud (Fig. 95, kz"). Then 

 we find the birthplace of the germ-cells dill further back (Fig. 95, kz""), 

 for the Qgg and sperm-cells arise in the stem of the principal polyps 

 (the main stem of the colony). The advantage of this arrangement 



